1. Field of the Invention
This invention relates to a tracking control arrangement and, more particularly, to the use of fuzzy inference, or fuzzy reasoning, to achieve highly accurate tracking control.
2. Description of the Prior Art
Tracking control commonly is used when reproducing previously recorded signals from a record medium wherein the medium or the playback transducers (or heads) or both are movable. For example, automatic track finding (ATF) is used conventionally in many video tape recorders to make certain that the playback heads accurately trace the tracks in which video signals have been recorded. ATF control is attained by recording pilot signals whose frequency alternates between reference frequencies from track to track, whereby the relative magnitudes of these pilot signals picked up during a playback operation are compared to provide an indication of a tracking error. The deviation of the head from the track being scanned is related to the magnitude of the pilot signal picked up from an adjacent track and, of course, the track from which a pilot signal is picked up is readily discriminated by the pilot signal frequency. While ATF control is advantageous because it achieves automatic tracking adjustments without the need for user intervention, a typical ATF control circuit is relatively complicated and expensive.
An alternative to ATF control relies upon the signal level, such as an integrated value, the envelope or the long term average level, of the reproduced signal as an indication of a tracking error. As described in, for example, Japanese Patent Application No. 245507/1988, the transport speed of the magnetic video tape is controlled in a manner which maintains this signal level above a predetermined threshold. For example, as shown in FIG. 1 herein, the signal level of the reproduced signal is a maximum E.sub.MAX when the playback head is disposed at the center of the track in which the signal is recorded; but the level of the reproduced signal E(n) decreases as the head deviates from the track center. Tracking control is achieved automatically by accelerating or decelerating the magnetic tape in response to the detected signal level E(n) which, as shown, is related to the tracking error. The deviation of the signal level E(n) from the maximum signal level E.sub.MAX is obtained simply by comparing the level of the reproduced signal E(n) to the preset maximum level E.sub.MAX. The resultant error signal K[E.sub.MAX -E(n)] is fed back to the tape transport mechanism as a tracking control signal to correct for detected tracking errors.
While the foregoing tracking control arrangement is relatively simple and achieves tracking correction automatically, various disadvantages arise with respect thereto. For example, the actual maximum signal level E.sub.MAX which can be reproduced from a particular record medium may be substantially less than the preset value E.sub.MAX. This frequently occurs when signals are recorded on a magnetic tape by one video recorder and reproduced by another having different characteristics. When the actual maximum signal level is less than the preset level E.sub.MAX, a proper tracking condition will not be detected because the error signal K[E.sub.MAX -E(n)] will not be reduced to zero even when there is no tracking error. Consequently, in an attempt to achieve tracking correction, oscillations may be generated.
Another disadvantage is attributed to the assumption in the tracking control arrangement that the error signal K[E.sub.MAX -E(n)] is linear. However, as is appreciated from FIG. 1, changes in the signal level E(n) reproduced from the magnetic tape vary in a non-linear manner with tracking errors. Consequently, and because of this non-linearity, the sensitivity of the tracking control arrangement may increase for large tracking errors but may decrease for small tracking errors. As a result, both transient and steady state response characteristics of this tracking control arrangement may be deficient.